Method of immobilizing contaminants in the soil or in materials similar to the soil

ABSTRACT

A method of immobilizing a contaminant comprising mixing the contaminant with a reaction partner that is capable of chemically interacting with the contaminant to form a water-insoluble reaction product, the reaction partner being mixed in the form of a hydrophobic solid preparation, which is either obtained by grinding the reaction partner with an inert material and treating it with a hydrophobing agent or which contains the educt or reaction product of a dispersion by chemical reaction preliminarily treated with a hydrophobing agent, the mixing being conducted to form a soil or soil-like material with cohesive constituents of a clay-like structure.

This application is a continuation of application Ser. No. 328,253,filed Mar. 16, 1989 and now abandoned.

Converting contaminants that pollute the soil or materials similar tothe soil into materials that are ecologically inert in relation to theenvironment by means of chemical interactions with appropriate reactionpartners are known. Such contaminants can be natural or man made. Potslick, milk of lime, and sedimentation from engineering plants areexamples.

These contaminants are either organic or inorganic compounds. Theorganic compounds can be substituted or unsubstituted, saturated orunsaturated, and aliphatic or aromatic hydrocarbons with or withoutheteroatoms. The inorganic compounds can for example be cations, inheavy-metal compounds for example, or anions, cyanides for example. Allconstitute a potential ecological hazard.

The chemical interactions with appropriate reaction partners are forexample adsorption, oxidation and reduction, ion exchange, complexing,precipitation, condensation, polymerization, etc.

Examples of the conversion of a contaminant that is potentiallyhazardous into a material that is ecologically inert in relation to theenvironment by means of chemical interaction with an appropriatereaction partner are the polymerization of monomers or oligomers in thesoil by adding polymerization catalysts, the precipitation of heavymetals in the form of hydroxides in a slime by adding hydroxyl ions, andthe adsorption of contaminants in sewage sludge onto sawdust, activecarbon, ground bark, etc. Another chemical interaction is thedissolution of a contaminant in a solvent, dissolving chlorobenzene inan asphalt that has been finely divided by chemical reaction in calciumhydroxide and exists in powdered form (dispersion-by-chemical-reactionpulverized asphalt) for instance. The reaction partners are, as will beevident from the examples, added in the form of an aqueous solution, asuspension, or a solid.

This method has several drawbacks. The aqueous solution of a reactionpartner can for example be fully effective in a moist soil only subjectto certain soil-mechanical conditions.

A homogeneously sandy soil contaminated with heavy metals can be treatedin the present context for instance by spraying it with an aqueoussolution of a sulfide. It is known that heavy-metal sulfides arepractically insoluble and are accordingly ecologically inert in relationto the environment. As the solution seeps into the soil, then,practically all the heavy-metal ions in the sandy soil are reached bythe reaction partner, the sulfide ion, and precipitate out in the formof heavy-metal sulfide.

Sometimes, on the other hand, the soil contains cohesive constituents.In the present example, that is, the sand can be interspersed withcoarse or fine clay. Such substances will prevent the heavy metals fromreacting with the sulfide ion in the aqueous solution and the metalswill remain just as potentially hazardous to the environment as ever.

Whether the contaminated soil is treated in situ or after being dumpedinto appropriate mixing equipment is insignificant because plastic clodsof coarse and fine clay will persevere very extensively throughout themixing procedure.

There exists an additional hazard. The added sulfide ions that cannotget to the heavy-metal ions for the aforesaid reasons and areaccordingly themselves prevented from interacting, arrive in thegroundwater and become active as a contaminant with a considerablehazardous potential.

The significance of this example can of course be transferred to theother methods of making contaminants inert by means of appropriatereaction partners. Once a contaminated material contains coarse or fineclays, the aforesaid known concept of converting contaminants intomaterials that are ecologically neutral will fail for the foregoingreasons even though the chemical principles have been thoroughlyinvestigated. These remarks also apply to such other aqueouspreparations as suspensions and emulsions. If the aforesaidsoil-mechanical conditions are in force, no all-inclusive reactionbetween the contaminant and its intended reaction partner can occur.

The chemical reaction partner can be added in the form of a solid orsolid preparation to a moist soil. Such solids are for example sawdust,hydraulic binders, etc. In this case, the solid, which can absorb water,will be observed to cake up and become lumpy, preventing the homogeneousdistribution of the actual reaction partner contained therein throughoutthe moist soil. This situation will be even more likely to occur whenthe composition of the material being treated is inhomogeneous, when,that is, it contains structures of the nature of coarse or fine clay.

Finally, there is still another and quite considerable drawback. Theconstituents of aqueous or solid preparations of reaction partners andof solid reaction partners that can be mixed or soaked with water canseparate when the substances are introduced into a moist material thatis to be treated. The phenomena are similar to those that occur inchromatography, with the individual constituents of the systemcontaining the reaction partners being separated to the extent that theycannot act together effectively at the site of the contaminant.

German A 2 434 879 discloses the use of thiuram-sulfide compounds toremove heavy metals from an environment polluted by them. The process isespecially applicable to sewage, exhaust gases, polluted rivers andseas, and polluted terrain. When the process is applied to soil that hasbeen polluted with heavy metals, the heavy-metal compounds must beleached out and transferred to aqueous systems before they can beprecipitated in the form of water-insoluble reaction products.

In methods with the characteristics recited in the preamble to the majorclaim of German A 24 34 879, however, a contaminant that has beenconverted into a water-insoluble reaction partner is to be retained inthe soil. This would be impossible with the known heavy-metal removerbecause the resulting water-insoluble reaction products are sensitive tohydrolysis. The thiuram-sulfide compounds behave no differently in thesoil than other known reaction partners known for converting heavymetals. The specific problems that occur with a soil or soil-likematerial that contains cohesive constituents, especially structures likecoarse or fine clay, cannot be solved by the teachings of theaforementioned document.

The object of the invention is accordingly to provide a method thatlacks the aforesaid drawbacks and that will ensure the effectiveness ofthe known chemical interactions between contaminants and appropriatereaction partners even in moist, inhomogeneous, and cohesive structures,leaving the environment unaffected by reaction partners that areprevented from interacting.

This object is attained in accordance with the invention in a method ofimmobilizing contaminants in the soil or in soil-like materials whereina reaction partner that is capable of interacting chemically with thecontaminant to form a water-insoluble reaction product is worked in theform of a hydrophobic solid preparation into the soil or soil-likematerial.

Hydrophobic solid preparations are defined for the present purpose assolids that have been treated with a hydrophobing agent and optionallywith other auxiliaries as well.

Appropriate hydrophobing agents are known. Biodegradable agents arepreferably employed. These are for example such natural fatty acids asstearic acid and palmitic acid for instance and such alkanes as paraffinoil for instance.

Enough hydrophobing agents are employed to ensure that, once the finelydispersed hydrophobic solid preparation has been mixed into the soil orsoil-like materials, the reaction partners will be released from theinterface within a foreseeable time. Depending on the type ofhydrophobing agent, it will constitute between 0.1 and 5% of the totalmixture. When longer-chain paraffins, alcohols, and carboxylic acids areemployed as hydrophobing agents, they will preferably constitute 1% ofthe total. It is also possible to add up to 20% of such inorganichydrophobing agents as talc, depending on the solid being hydrophobed.

Solids that can be employed for the purposes of the invention aresubstances with chemical compositions that allow them to be mixed safelyinto the soil. These substances can be such inert inorganic solids asfinely dispersed silicic acid, powdered limestone, or calcium hydroxideor such silicates as bleaching clay or bentonite.

There are also many organic materials that can be employed as practicalinert substances in this context. These materials are ground bark,sawdust, powdered cellulose, active carbon, macromolecular synthetics,solid paraffin, wax, polyethylene glycol, asphalt, etc.

Finally, mixtures of such inorganic and organic constituents as usedbleaching clay from the foodstuffs industry can also be employed.

The hydrophobic solid preparation will preferably include the educt orreaction product of a chemical dispersion reaction. Dispersion bychemical reaction, the DCR reaction, is a simple method developed by thepresent inventor of dividing liquids and solutions of solids or liquidsaccompanied by the formation of extensive surfaces by means of chemicalreaction. DCR is the object of German Patents 2 053 627, 2 328 777, 2328 778, 2 520 999, 2 533 789, 2 533 790, and 2 533 791, and theirforeign equivalents. There are a great many chemical reactions thatextend surfaces in the foregoing sense and are accordingly appropriatefor chemical dispersion. Especially worth mentioning are the conversionof calcium oxide with water into calcium hydroxide and the hydrolysis ofaluminum alcoholates into aluminum hydroxide.

A preferred educt for the DCR method is calcium oxide in thecommercially available form of quicklime, in the fine-white chalk gradefor example. Coarse grades, however, are also usable in many instances.The quicklime can contain up to 18% of magnesium oxide or other foreignmatter by weight.

The DCR method can be carried out by preliminarily dispersing thehydrophobing agent, a reaction partner that is capable of chemicallyinteracting with the contaminant to form a water-insoluble reactionproduct, and optionally other auxiliaries throughout the DCR educt andthen allowing the surface-extending reaction to occur. The result willbe finely dispersed solid preparations with the added compoundsexhibiting an especially powerful chemical reactivity.

When contaminants in the form of heavy-metal salts are to be treated,the reaction partner that is capable of forming a water-insolublereaction product is a soluble sulfide, hydroxide, carbonate, orphosphate. Sodium sulfide or calcium sulfide for example can beemployed. The latter can be obtained by the DCR method from sulfur incalcium oxide.

In one preferred embodiment, accordingly, calcium oxide is treated,ground for example, with hydrophobing agents and reaction partners andthe resulting finely dispersed hydrophobic solid preparation is mixedinto the soil or soil-like materials. The calcium oxide will react,although slowly due to the hydrophobing, into calcium hydroxide, whichwill contain all the added substance is a finely dispersed andaccordingly powerfully reactive form.

Chemical reactions of the type described herein occur in aqueoussolution. It is accordingly surprising to discover that the reactionpartners in the solid preparation will also be released in the soil orsoil-like materials when the individual solid particles are hydrophobic.

Just the use of a finely dispersed solid preparation that has beenhydrophobed makes it possible to mix it even into moist soil orsoil-like materials. The water needed for the dispersion reaction isextracted from the moist soil. Even soils that contain coarse or fineclay can accordingly be homogeneously penetrated by the solid, althoughthe mixing process must last longer. The water of reaction can also beobtained from outside in the form of a reaction partner that isappropriate for immobilization, red mud for example.

How the precipitated heavy-metal compounds behave in the soil depends onthe structure and constituents of the soil and on its pH andtemperature. Metal sulfides can be microbially oxidized into solublesulfates at low pH's. Even assuming that an initially present calciumhydroxide will be eventually completely converted into carbonate, themicrobial activity will still be inhibited to an extent that can beincreased by codispersing agents that suppress the activities of theThiobacillus strains.

All of the amounts cited in the following examples are by weight.

EXAMPLE 1

1 part of a sandy soil contaminated with approximately 500 000 ppb ofhexachlorobenzene in terms of the dry volume was thoroughly mixed with0.5 parts of a mixture, hydrophobed by milling with 2% of a mixture ofnatural fatty acids, of 0.7 parts of calcium hydroxide and 0.3 parts ofused bleaching clay obtained from the vegetable-oil processing industry.Samples compressed from 50 g of the mixture at 24.5·10⁵ Pa and subjectedto a leaching test in accordance with German Unit Process (DEV) S4 asnecessitated by the method still yield 0.1 ppb of hexachlorobenzene.This level, however, decreases rapidly to zero over three additionalleaching steps due to the impoverishment of the contaminant at thesurface of the sample.

Instead of the aforesaid mixture, 0.5 parts of a hydrophobic reactionmixture from a dispersion by chemical reaction (DCR) involving 0.5 partsof calcium oxide, 1% of the same mixture of fatty acids, and 0.2 partsof used bleaching clay of the same provenance can be mixed with the soiland with 1.5 times the requisite stoichiometric volume of water. Whenchlorinated dibenzodioxines are in the soil, it is of advantage toreplace the water for the dispersion by chemical reaction with theestimated volume of red mud containing approximately 40% water.

EXAMPLE 2

1 part of a sandy soil contaminated with approximately 500 000 ppb of acontaminant mixture of tri- and tetrachlorobenzolene and penta- andhexachlorobenzene in terms of the dry volume was thoroughly mixed with0.5 parts of a hydrophobic reaction mixture obtained from a dispersionby chemical reaction involving 0.5 parts of calcium oxide with 1% of amixture of natural fatty acids and 0.4 parts of molten Type B 80 asphaltand with 1.2 times the requisite volume of water. Samples compressedfrom 50 g of the mixture at 24.5 10⁵ Pa and subjected to a leaching testin accordance with German Unit Process (DEV) S4 as necessitated by themethod yield no detectable levels of contaminants subject to otherwiseidentical analytic conditions.

Instead of the aforesaid reaction mixture obtained from the aforesaiddispersion by chemical reaction, it is possible to use a mechanicallyobtained hydrophobic mixture of the same volume of the same asphalt with1.5 parts of powdered limestone and optionally with 1 part of a flue ashwith hydraulic properties. It is also possible to used appropriateasphaltic wastes instead of the B 80 asphalt. It may be necessary to addother inert materials, in the form of the sandy soil itself for example,to obtain a homogeneous powdered preparation.

One especially simple version of the method consists of grinding asphaltand/or appropriate asphaltic wastes, optionally in liquid form, with anuncontaminated sand at a volumetric ratio that will result in aperceptibly dry preparation that can easily be worked in the sense ofthe object of the invention and of mixing the preparation with thecontaminated sandy soil in a volumetric ratio that will result in amaterial that can be satisfactorily compacted in accordance withsoil-mechanical criteria.

EXAMPLE 3

1 part by weight of a cohesive soil inhomogeneously contaminated with acertain percentage of an unidentifiable mixture of contaminants obtainedfrom the production of different vegetation-protection agents andcontaining bound mercury in addition to other heavy metals is treatedwith 0.8 parts of the following mixture.

1 part of B 80 asphalt wa dispersed by chemical reaction with 1 part ofcalcium oxide and treated with 0.4 parts of powdered limestone enrichedwith 5% of a finely dispersed calcium sulfide. Compaction results in asoil that is soil-mechanically stable. Samples compressed from 50 g ofthe mixture at 24.5·10⁵ Pa and subjected to a leaching test inaccordance with German Unit Process (DEV) S4 as necessitated by themethod yield no detectable levels of contaminants subject to otherwiseidentical analytic conditions or when analyzed for mercury and copper ororganic compounds containing chlorine.

To control these results, up to 1 million ppb of hexachlorobenzene andHg²⁺ each were added to reference batches of the same contaminatedcohesive soils. Neither contaminant could be detected in leaching testssubject to otherwise identical conditions. The same result is obtainedwhen the sulfur is dispersed by chemical reaction with calcium oxidewith molten asphalt and the same procedure is followed.

The dispersion by chemical reaction can also be replaced in this exampleby such mechanical procedures as grinding for example.

EXAMPLE 4

A mixture is produced from the same number of parts of hydrophobiccalcium oxide, into which 1% of calcium sulfide has been ground, and ofground slag obtained from a smelter. 0.8 parts of this mixture arethoroughly mixed with 1 part of a pot slick preliminarily dewatered in acentrifuge and containing organic and inorganic contaminants, especiallymercury, arsenic, lead, and cadmium. Upon termination of the dispersionby chemical reaction and subsequent to cooling, the soil-like reactionproduct can be compacted to make it appropriate from the aspect of soilmechanics and exploited in landscaping for example. Samples compressedfrom 50 g of the mixture at 24.5·10⁵ Pa and subjected to a leaching testin accordance with German Unit Process (DEV) S4 as necessitated by themethod still release up to 12 ppm of heavy metals, especially lead. Asthe surface of the sample continues to carbonatize, however, theconcentration of heavy metals drops rapidly to zero.

The same result can be attained by using calcium oxide in the form ofsecondary raw materials obtained from desulfurization plants instead ofthe commercially available substance or by replacing the powdered slagwith used bleaching clay. When samples of the aforesaid composition areembedded in an isolating layer of asphalt dispersed by chemical reactionin calcium hydroxide, asphaltic wastes, or plastic wastes, no detectableconcentration of heavy metals or of whatever other problematic organicmaterials may be present are released in even one leaching step.

It will be understood that the specification and examples areillustrative but not limitative of the present invention and that otherembodiments within the spirit and scope of the invention will suggestthemselves to those skilled in the art.

What is claimed is:
 1. A method of immobilizing a contaminant containedin contaminated soil comprising mixing the contaminated soil with areaction partner selected from the group consisting of a solublesulfide, hydroxide, carbonate and phosphate, and which is capable ofchemically interacting therewith to form a water insoluble reactionproduct, the reaction partner being part of a hydrophobic solidpreparation which is obtained bya) grinding the reaction partner with aninert material and mixing it with a hydrophobizing agent selected fromthe group consisting of talc, a biodegradable natural fatty acid, longchain paraffin and an alcohol, or b) grinding the reaction partner withthe product of a dispersing chemical reaction (DCR) selected from thegroup consisting of calcium hydroxide and aluminum hydroxide and mixingit with a hydrophobizing agent as recited above,the mixture of thecontaminated material with the hydrophobic solid preparation forming aninert solid.
 2. A method according to claim 1, wherein the solidpreparation is hydrophobized with reaction product of a DCR with ahydrophobic educt.
 3. A method according to claim 1, wherein the solidpreparation has been hydrophobized in the course of a dispersingchemical reaction.
 4. A method according to claim 1, wherein the solidpreparation contains the reaction partner in the form of the educt of adispersing chemical reaction.
 5. A method according to claim 1, whereinthe solid preparation contains the reaction partner in the form of thereaction product of a dispersing chemical reaction.
 6. A methodaccording to claim 1, wherein the solid preparation contains thereaction partner embedded in an inert organic material.
 7. A methodaccording to claim 1, wherein the dispersing chemical reaction iscarried out with calcium oxide or aluminum alcoholate.
 8. A methodaccording to claim 1, wherein the solid preparation contains a reactionpartner that immobilizes by precipitation, condensation, dissolution,complexation or addition reactions to form water insoluble reactionproducts.
 9. A method of decontaminating a contaminated soil whichcomprises immobilizing the contaminants according to the method of claim1 and subsequently removing the inert solid formed.